Bulk delivery systems are used, for example, to supply process equipment in the pharmaceutical, cosmetic, and semiconductor industries with blended process materials such as liquid chemicals or slurries. In the semiconductor industry, blended process materials are typically prepared using batch production systems including a blending subsystem and a distribution subsystem. In the blending subsystem, process materials are added to a blending vessel or tank. For example, the process materials might consist of a solid material, such as a dry powder containing abrasives or chemical compounds, and a liquid, such as deionized (DI) water.
Many conventional processes require precise addition of process materials to produce a batch of blended process materials that is acceptable for its intended application. Accordingly, the measuring instruments that monitor the inputs to the blending tank are typically very precise to insure batch-to-batch consistency. In many applications, even minor process variations may lead to significant differences in the batch of blended process materials, potentially rendering it useless for its intended application.
FIG. 1A shows a typical prior art arrangement of a solids blending subsystem 100 for mixing a process material. Solid feeder 10 holds a supply of dry solid material in a hopper 11. The dry material is moved from the Solid feeder to blending tank 12 by using a feed screw mechanism. The amount of solid material added to the blending tank 12 may be controlled via a loss-in-weight mechanism 13. The solid feeding element makes use of a very accurate scale to determine when the precise amount of dry material has been fed into the blending tank based upon the weight of the material remaining in the hopper of the solid feeding element 10. The solid material is often mixed with a liquid in the blending tank 12 via an impeller or mixer 14 to form a homogeneous batch of blended process materials. Tank may also include an agitator, such as an impeller or a sparger head 15, particularly where the process materials may settle or separate.
Once the process materials have been adequately mixed, the process material is typically transferred through the material supply lines to the distribution subsystem 102, shown in FIG. 1B. The blended process material is first pumped to a distribution tank 22, commonly referred to as a day tank. The process material can then be distributed via a distribution pump 27 from the day tank 22 through the global loop 24, 25 to the final points of use 28 for its intended application. Points of use may be any location where there is demand for a supply of the blended process material. For example, points of use may include process machinery or work stations in a semiconducting fabrication facility.
As also shown in FIG. 1A, the process material can be pumped from the blending tank to the distribution tank by dual diaphragm pump 17, although other types of pumps could also be used. A typical bulk delivery system may also a process control system including flow control devices or sensors positioned on the material supply lines 16 and a controller comprising logic code to provide a control signal to the flow control devices based upon sensor signals (not shown). The movement of process material through the material supply lines can be controlled by a variety of known flow control devices, including manual valves 18, automatic diaphragm valves 19, or three-way valves 20 (which also allow air to be vented from the system). In some cases, a pulsation dampener 21 may be employed to smooth the pressure in the lines resulting from the transfer pump 17.
Prior art bulk material delivery systems are described, for example, in U.S. Pat. No. 7,344,298 to Wilmer et al. for “Method and Apparatus for Blending Process Materials” (Mar. 18, 2008) and in U.S. Pat. No. 6,923,568 to Wilmer et al. for “Method and Apparatus for Blending Process Materials” (Aug. 2, 2008), both of which are incorporated herein by reference.
There are a number of disadvantages to this type of prior art bulk delivery system. First, the blending process itself suffers from some inherent inefficiency. Significantly, as the solid material from the solid feeding element is added to the liquid in the blending tank, the contact area for wetting the added solid material is limited to the surface of the liquid in the tank. This slows the rate at which solid material can be added to the tank. The localized forced-convection mixing of the solids into the liquid in a prior art system can also be relatively inefficient for fine, hydrophobic solids, due in part to the tendency of some solid materials to float on the surface of the liquid rather than being entrained into the circulating liquid in the blending tank.
Also, where only one blending tank and one distribution tank are employed, it is often necessary to “spike” the mixture with either solids or liquids at the distribution tank to maintain product character while the blending system is producing another batch. For this reason, it is more desirable to use multiple solids feeders. The solid feeder is quite expensive when compared to the other components, and the prior art system requires a solid feeder for each process material tank used for blending or mixing. The cost associated with a solid feeder thus increases in one-to-one multiples with the number of active blending tanks and distribution tanks if spiking functions are present. Further, where multiple blending tanks are employed separate set-up is required to tune each solid feeding element to achieve batch-to-batch reproducibility within a given system.
What is needed is an improved method and apparatus for process material blending that provides an increased wetted surface area for liquid contact as well as increase forced convective mixing efficiency of the mixture. What is also needed is an improved apparatus that allows one solid feeder to serve multiple tanks to reduce overall costs and increase operational efficiency.